This invention relates to a method of, and a photographic system for, exposing a recording medium to a light image. More specifically, the invention relates to a system which prints computer generated, or computer stored images or information onto light sensitive film or material while maintaining a high resolution and geometric accuracy in the printed image. The system also provides the capability to read or digitize film-based imagery or information with the same high resolution and geometric accuracy.
In general, most commercial film printers consist of a cathode ray tube (CRT), a lens, a film holder, and related drive/control electronics. An image displayed or drawn on the CRT faceplate is projected by the lens onto the film surface, and is thereby printed. Color images are frequently printed by decomposing the original image into three primary color frames and sequentially printing each frame through an appropriate color filter onto a single piece of recording media.
Other image characteristics, such as shading information, may incorporated into the final printed image. U.S. Pat. No. 3,852,782, issued Dec. 3, 1974 to Gundlach et al., describes an imaging system wherein the light in contiguous sections of an image is differentially attenuated and focused through a select one of adjacent lenticules onto contiguous segments of a photoreceptor. This provides for the recording of highlights, medium tones, or shadows in the image produced on the photoreceptor.
It has been another object in the art to provide for the recordation of multiple images. U.S. Pat. No. 4,115,002, issued Sept. 19, 1978 to Clark, discloses a device whereby separate total images are selectively overlapped for integrating these individual images into a resulting final copy having a high information ratio. The images are projected onto the film plane through various lenses positioned in a predetermined manner. Provision is also made for color and density correction filters for altering the color balance and density, respectively, of the original images.
U.S. Pat. No. 3,703,135, issued Nov. 21, 1972 to Lang, also describes a multiple image film exposure and projection system. This system successively exposes small areas of a film frame through a shuttered lens array, thereby providing a plurality of individual images on the frame without the need to move either the camera or the frame.
A method of obtaining a double image of a single object is disclosed in U.S. Pat. No. 4,088,401, issued May 9, 1978 to Rees et al. In this method, multiple projection lenses with shutters are also used to achieve the double imaging.
The image on a CRT faceplate is created by deflecting and modulating an electron beam within the CRT vacuum envelope. As the electron beam strikes the inner wall of the CRT faceplate, a layer of phosphor converts the electron beam energy to light. The resolution of the CRT image depends on the electron beam spot size and shape, the grain size of the CRT phosphor, and the degree to which the beam deflection system can accurately and reproducibly address an absolute position on the CRT face. The geometric accuracy of the CRT image, while not specifically dependent on the CRT resolution, can equally affect the resolution and accuracy of the printed image, as can distortions introduced by the projected lens. The higher the resolution and geometric accuracy of the CRT image source, the more detailed and accurate will be the final printed image.
A major drawback in obtaining high resolution and accuracy in prior art film printers has been cost. In general, there is rarely an exactly known and fixed relationship between deflection system commands and resulting CRT beam position. Although this relationship may at some point in time be measured or calibrated, the varying of environmental factors such as temperature and magnetic field strength tend to reduce the long term accuracy and reliability of such information. Components and systems resistant to such changes are costly and imperfect. Consequently, the CRT image and resulting film print is usually only an approximation to the initial image data since the individual points of the image are never perfectly located.
Such considerations apply especially in the case of printing a color image. In order to insure correct registration of the three separate primary color frames, the film printer must maintain a high precision over the entire time required to print all three frames. Thermal drift of the beam deflection electronic parameters make such registration difficult.
One way to achieve a higher CRT display resolution is feedback, whereby inaccuracies are continually corrected. One such system, which utilizes a feedback arrangement to correct CRT display image distortions due to vibrations is disclosed in U.S. Pat. No. 4,630,115 issued Dec. 16, 1986 to Hilsum. In particular, this device directs light from a spot on the CRT screen onto a photocell which detects the position of the light spot. Feedback is then utilized to process the output of the photocell detector to provide a correction signal which is, in turn, utilized by the deflection system of the CRT to adjust the position of the electron beam and, hence, the light spot. This correction is applied to the image as a whole in order to reduce the net motion of the image. Individual points or pels within the image are not adjusted relative to one another, and the method does not enhance the accuracy or precision of the displayed image.
As discussed above, prior devices involve the use of single element photo-sensors. Moreover, where a photo-sensor is used to feed back CRT beam position information, the CRT beam must be moved to the periphery of the CRT surface in order to excite the sensor. The accuracy of the final, determined position of a light spot depends upon the precision of the photodetector system.
Deflection system calibration information derived from such beam position measurements becomes progressively less accurate in regions away from the point of calibration. Deflection corrections which apply to beam positioning in the interior of the image must be extrapolated from these peripheral data and are consequently less accurate. The effective measuring range of a photosensitive element can be extended by enlarging the sensor area of the element. With this technique, however, it is increasingly difficult to ensure homogeneity over the entire enlarged sensor area. In addition, it becomes prohibitively expensive to provide further system components of sufficient precision.
U.S. Pat. No. 4,457,626 issued July 3, 1984 to Idesawa et al., describes an alternative type of device for more accurately determining positioning information. More specifically, this device includes a single lens in operative association with a mirrored cavity for reflecting a beam of light from a designated mark on an object onto a photosensitive element. The particular system used thereby, effectively enlarges the area of the photosensitive element. Idesawa et al.'s device uses a feedback algorithm which calculates positioning information based on the beam's detected position on the photosensitive element, the location of the object or spot source, and the number of reflections of the light beam within the mirrored cavity.
One disadvantage of this prior art device is the loss of intensity of the light beam incurred during reflection of the beam from the walls of the mirrored cavity. In addition, the walls of the mirrored cavity must be precisely parallel and flat to eliminate distortions which would otherwise occur. Also the size of the collection lens is limited to the physical size of the photosensitive element, because the mirrored walls must begin just at the edge of the photosensitive element and must encompass the collection lens.
A further disadvantage with prior art devices is that they can only correctly image surfaces of uniform curvature. Moreover, absolute position calibration, absent some knowledge of the history of a spot's motion, has heretofore no been easily achieved.
Thus, there exists in the art a genuine need for a film printing system which achieves a high precision and accuracy over the entire image area by the effective use of feedback for correct beam placement.
Furthermore, there exists a need for an inexpensive method of enhancing the effective area of existing photo-sensors without introducing distortions or limiting the available light.
Lastly, a need exists for the ability to precisely locate the CRT beam to provide for the accurate positioning and joining of adjacent image sections projected onto the recording film medium, thereby producing a single, high quality image that has a significantly higher resolution than the single CRT image source. Prior art does not disclose success in this process of merging image sections to reproduce a larger, original, single image.